The long-term goal of this proposal is to identify the E series of prostaglandin (EP) receptor subtypes involved in mechanical loading- induced bone formation (LIBF) in vivo. Mechanical loading stimulates bone formation both in vitro and in vivo via prostaglandin (PG) E2 and nitric oxide pathways. Treatment with PGE2 stimulates both bone formation and resorption in vivo. There are four subtypes of EP receptors, the EP1-4. These subtypes are coupled to different (combinations of) second messengers, which may explain the diverse effects of PGE2. Our preliminary studies show that treatment with SC19220, an EP1-specific antagonist, inhibits LIBF of rat tibiae in vivo. The EP4-specific antagonist AH23848B had no effect. This suggests an involvement of the EP1 subtype in LIBF. These findings imply that EP1 agonists may be used to prevent and treat osteoporosis more effectively with less side effects. This proposal will further investigate the mechanisms whereby EP1 receptors mediate bone formation induced by loading. The role of SC19220 in inhibiting LIBF in vivo will be first established pharmacologically. To determine whether SC19220 inhibits periosteal osteoblast-like cell (OBC) proliferation in vivo in response to loading, rats will receive a single injection of SC19220 three hours before four-point bending. Three days later, the number of periosteal OBC at the tibiae will be examined. To determine whether EP1-specific agonists stimulate bone formation in vivo, rats will be treated with 17-w-phenyl-trinor-PGE2, an EP1-specific agonist, for four weeks followed by histomorphometric examination of tibial bone formation rate. To investigate whether EP1 receptors directly stimulate osteoblast proliferation in vitro, 3H-thymidine incorporation into ROS 17/ 2.8 cells will determined with the cells cultured in the presence of the EP1 agonist.